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Posted: Jul 19th, 2007
Nanotechnology could clean up the hydrogen car's dirty little secret
(Nanowerk Spotlight) Back in January, when the U.S. president announced his hydrogen fuel initiative and proposed to spend a total of $1.7 billion over the next five years to develop hydrogen-powered fuel cells, hydrogen infrastructure and advanced automotive technologies, he said that it will be practical and cost-effective for large numbers of Americans to choose to use clean, hydrogen fuel cell vehicles by 2020. According to the U.S. Department of Energy's (DOE) Hydrogen Program, the government's goal is to achieve "technology readiness" by around 2015 in order to allow industry to make decisions on commercialization by then. That's only eight years to go. Given where the technology is today, this goal seems very ambitious, to say the least. Nanotechnology could help speed up the journey to the hydrogen society, but it will take some sensational breakthroughs on the way. The three key areas for the vehicles (we will not touch on the infrastructure issues here) are clean - the emphasis is on clean - hydrogen production, hydrogen storage, and the fuel cell itself. We'll take a look at how nanotechnology will play a role in these areas.
First, let's get the terminology straight. Getting an internal combustion engine to run on hydrogen is not difficult (but it is difficult to get it to run smoothly). Some of the Hydrogen Vehicles on the road today still have an internal combustion engine, but one that uses either pure hydrogen or a mix of hydrogen and natural gas. True Hydrogen Fuel Cell Vehicles basically are electric cars (having a flashback to the 1970s here?) where fuel cells convert the chemical energy of a fuel – hydrogen – directly into electricity without any intermediate thermal or mechanical processes. Neat thing is that the exhaust consists solely of heat and water.
Hydrogen fuel cells get their hydrogen either produced on-board by converting liquid fuels (gasoline, ethanol, or methanol) to hydrogen, or by using hydrogen that has been generated off-board and stored on the vehicle. Where that off-board generated hydrogen comes from is problem number 1: There are no hydrogen wells.
Hydrogen has to be produced, and that can be done using a variety of resources. The cleanest by far of course would be renewable energy electrolysis: using electricity to split water into hydrogen and oxygen; this electricity could be generated using renewable energy technologies such as wind, solar, geo- and hydrothermal power. The dirtiest, at least until highly efficient carbon capture and sequestration technologies are developed, is the gasification of coal. Of course you can also use nuclear energy to provide the electricity for electrolysis.
However, 95% of all the hydrogen produced in the United States today (and 50% worldwide), some 9 million tons annually, is produced from methane in natural gas using high-temperature steam – so-called steam methane reforming. Government researchers say that they see natural gas only as a 'near-term' solution; 'near-term' meaning the time it takes to come up with a better and cleaner solution that scales industrially. That solution doesn't exist yet.
And here is the dirty little secret: while politicians and the energy industry talk about the clean future of the hydrogen economy, the DOE's Hydrogen Energy Roadmap foresees up to 90% of hydrogen production coming from fossil fuels – coal, gas, oil – the rest mostly from nuclear power plants (why do you think the oil companies are investing hundreds of millions of dollars into hydrogen technology?). In other words: although hydrogen fuel cell cars themselves may emit nothing but water and heat, the process of powering the fuel cells with hydrocarbons will continue the economy's dependence on fossil fuels and leave behind carbon dioxide (sequestered or not), the primary cause of global warming.
(For an in-depth discussion of hydrogen energy read the policy statement "Renewable Hydrogen" (pdf download, 788 KB) from the American Solar Energy Society.)
The greatest challenge to clean hydrogen production is its cost. Unless government mandates the use of hydrogen or significantly increases the taxes on existing fossil-based fuels, the 'gallon/liter gasoline equivalent' (the amount of fuel with the energy content of one gallon/liter of gas) will be the measure used by drivers to decide what fuel to use. And the cheapest way today to produce hydrogen is from fossil fuels.
Nanotechnology's major contribution to the clean production of hydrogen lies in its application to solar cells and the catalysts used in water electrolysis. The holy grail here would be a highly efficient device that you fill with water, put in the sun, and get hydrogen without using any outside source of energy. Solar cells have the potential to make this dream come true. The two key issues for now are efficiency (which is low) and cost (which is high).
In one type of solar cell hydrogen is generated directly in a photoelectrochemical process that is based on the conversion of sunlight energy to chemical energy. It has been shown that nanoscale electrode materials, resulting in higher surface area to volume ratios, will increase the efficiency of the cell.
Another type of solar cell – a photovoltaic cell – produces electricity that can then be used to power electrolytic production of hydrogen from water. Experiments with nanowire arrays and other nanostructured materials have shown that they improve the efficiency of these cells.
Without going into details here – we have plenty of news articles and spotlights on this topic on our site – it is probably safe to say that nanotechnology will play an important role in building the type of highly efficient solar cells required to become a viable alternative to fossil fuel based hydrogen production.
Storing the hydrogen onboard that is needed to run your car's fuel cells poses another challenge. Very roughly speaking you would need about 1 kg of hydrogen to drive 100 km (or some 2.2 lbs. per 60 miles). That means you need about 5 kg/10lbs. of hydrogen to have the same average range as today's cars. Since hydrogen's density is only 1/10th of a gram per liter at room temperature, that means you somehow need to pack 50,000 liters (∼14,000 gallons) of hydrogen into your tank. There are three ways of doing this: as a high-pressure compressed gas; a cryogenic liquid; or as a solid.
Compressed hydrogen gas tanks will likely be used in early hydrogen-powered vehicles and will need to meet cost and packaging requirements to play a role across various vehicle platforms. Honda last year announced the FCX concept car that stores 5 kg of hydrogen at 5000 psi in a tank small enough to fit into a midsize car.
Rather than using thousands of psi to compress hydrogen into a tank, or cooling it down to minus 252°C (minus 421° F) to liquefy it, an intriguing alternative of hydrogen storage has led to metal hydrides, chemical hydrides, and physisorption-based storage, where hydrogen is adsorbed onto the interior surfaces of a porous material. The stored hydrogen can then be released by heat, electricity, or chemical reaction. Many metals are capable of absorbing hydrogen as well.
Nanotechnology plays an important role here. Nanomaterials have diverse tunable physical properties as a function of their size and shape due to strong quantum confinement effects and large surface to volume ratios. These properties are useful for designing hydrogen storage materials. For instance, researchers are now investigating nanostructured polymeric materials as hydrogen storage adsorbents. The new polymer adsorbent material has shown great promise in preliminary tests.
Due to their large surface areas with relatively small mass, single-walled carbon nanotubes (SWCNTs) have been considered very promising potential materials for high capacity hydrogen storage. Theoretically, they can store hydrogen up to 7.7 wt%, as every carbon atom in SWCNTs chemisorbs one hydrogen atom. In addition, the subsequent physisorption of hydrogen on the surfaces of hydrogenated SWCNTs can increase the capacity of hydrogen storage even further. However, there is some skepticism on carbon nanotube hydrogen storage due to early mistakes in experimental publications and a rational basis for high capacity hydrogen storage materials is now being developed.
Not surprisingly, a fuel cell is essentially just the reverse of an electrolytic cell: whereas electricity is used to decompose water into its constituent gases during electrolysis, in a fuel cell water and electricity are generated by the direct recombination of hydrogen and oxygen.
A major challenge for hydrogen powered cars today is the cost of the vehicle. The cost for fuel cells alone are currently hovering between $1,000 and $3,000 per kilowatt. To compete with vehicles equipped with internal combustion engines, those figures need to drop to about $30/kW. There are several kinds of fuel cells, but Polymer Electrolyte Membrane (PEM) fuel cells – also called Proton Exchange Membrane fuel cells – are the prime candidates for use in automobiles.
Both the electrolytic and the fuel cell use expensive platinum (which currently sells for about $45,000 per kilogram) as electrode material. Researchers are looking at two ways to bring the cost of catalysts down: One way to minimize platinum usage is to increase catalytic efficiency by nanostructuring the platinum metal; another way of eliminating the use of platinum altogether is by exploring the use of much cheaper non-precious metal catalysts where the nanostructured surfaces match or exceed the catalytic properties of platinum.
Nanotechnology certainly will play a major role in a future hydrogen economy. The big question is when most of this hydrogen will be generated from renewable sources and not fossil fuels. Certainly not by 2020.